1. Field of the invention
The present invention relates to an alumina sintered body and a spark plug having an insulator using the alumina sintered body. The alumina sintered body contains alumina crystals as a main phase, a crystal phase dispersed in the alumina crystals and a glass phase formed in a crystal boundary of the alumina crystals.
2. Description of the Related Art
Alumina sintered bodies are used as an insulator as an automobile component because of having a superior heat resistance and withstand voltage properties. For example, an alumina sintered body is used as an insulator of a spark plug used in an internal combustion engine of a motor vehicle. It is preferable for the insulator of the spark plug to be made of an alumina sintered body having superior withstand voltage properties. For example, patent document 1, Japanese patent laid open publication No. 2013-529355 has proposed a ceramic material containing alumina, oxide of group II alkaline earth metal and silicon dioxide.
Automobile fuels include alcohols such as ethanol in addition to fossil fuel such as gasoline and light oil. Because a conventional spark plug uses an insulator made of ceramic material, there is a problem of often causing corrosion of the insulator often by alkaline impurities such as Na (sodium) contained in alcohol fuel. Research and development of insulators used in spark plugs have been performed from one point of view to improve withstand voltage, but not performed adequately from the point of view of improving corrosion resistance of the spark plug.